In the case of my test blog Memcached isn't being used actively other than being installed as part of the default Centmin Mod web stack. Reason is PHP 7.0.0-dev doesn't support memcache and memcached/libmemcache PHP extensions right now, so no way for PHP-FPM to connect to Memcached servers. The benchmarks I did for the blog are pure Wordpress Super Cache plugin backed static file caching via Nginx web server = Nginx's real strength in static file serving